1. Field of the Invention
The present invention relates to a circuit simulator for performing a simulation of signal transmission in a transmission circuit in which a drive circuit which transmits a signal and a receiver circuit which receives the signal are connected to each other by a line, and to a circuit simulation program storage medium on which a circuit simulation program for enabling a computer to operate as the circuit simulator is stored.
2. Description of the Related Art
In the field of circuit designing, a circuit simulator for performing a simulation of a designed circuit on a computer before the circuit is actually manufactured is being used widely (see, for example, Japanese Patent Laid-Open Nos. 2000-331043 and 2002-197132). In the process of designing and manufacturing a transmission circuit in which a drive circuit which transmits a signal and a receiver circuit which receives the signal are connected to each other by a line, it is important to check the state of transmission of the signal before actually manufacturing the circuit. The circuit simulator is also used to check the state of transmission in such a case. Transient analysis for analyzing temporal changes of a signal on a line is ordinarily performed in a simulation of such a transmission circuit.
In some case of designing of a transmission circuit such as that described above, a technique is adopted to couple the driver circuit and the receiver circuit to each other by a line in which an AC coupling component such as a capacitor capable of blocking a DC current while allowing an AC current to pass therethrough is connected in series at an intermediate position in the line, for example, if a DC potential difference exists between the output end of the driver circuit and the input end of the receiver circuit.
FIG. 10 is a diagram showing a circuit model representing an example of a transmission circuit using a capacitor as an AC coupling component.
The transmission circuit represented by a circuit model 500 in FIG. 10 is a differential signal transmitting type of transmission circuit suitable for high-speed signal transmission.
The circuit model 500 shown in FIG. 10 has a device model 510 (hereinafter referred to as “driver model”) representing a drive circuit which transmits a signal, and a device model 520 (hereinafter referred to as “receiver model”) representing a receiver circuit which receives the signal. The two device models 510 and 520 are coupled to each other by two line models 530 and 540 respectively representing two lines. One of the two lines will be referred to as a Pos line, and the other of the two lines will be referred to as a Neg line. The line model representing the Pos line will be referred to as Pos line model 530, and the line model representing the Neg line will be referred to as Neg line model 540. In the transmission circuit represented by the circuit model 500, a signal is transmitted as a potential difference between the Pos line and the Neg line. The Pos line model 530 and the Neg line model 540 is a differential line model formed by also considering the coupling between the lines.
In the transmission circuit represented by the circuit model 500, a capacitor is connected in series as an AC coupling component in each of the Pos line and the Neg line. Accordingly, the Pos line model 530 is constituted by a pattern model 531 representing the pattern from the driver circuit to the capacitor, a device model 550 representing the capacitor (hereinafter referred to as “capacitor model”), and a pattern model 532 representing the pattern from the capacitor to the receiver circuit. Similarly, the Neg line model 540 is constituted by a pattern model 541 representing the pattern from the driver circuit to the capacitor, a capacitor model 550, and a pattern model 542 representing the pattern from the capacitor to the receiver circuit.
In this circuit model 500, an electrical direct connection is established between each adjacent pair of device models.
In the transmission circuit represented by the circuit model 500, a signal transmitted by the driver circuit is sent to the receiver circuit on the lines while any DC current is blocked.
In a transmission circuit having a capacitor as an AC coupling component for example, signal transmission is performed in a steady state in which the capacitor is sufficiently charged and stable in the charged state. However, if such a steady state is simulated by transient analysis, the time required for simulation by computation is considerably long. If analysis results are output before the transmission circuit enters the steady state, they indicate only a state during transition to the steady state, and differ from the actual signal transmission results.
FIG. 11 is a diagram showing analysis results of transient analysis before the steady state.
Part (A) of FIG. 11 is a Waveform showing changes in the potential on the Pos line model 530 and the potential on the Neg line model 540 during signal transmission. Part (B) of FIG. 11 is a Waveform showing changes in the differential potential, i.e., the difference between the potential on the Pos line model 530 and the potential on the Neg line model 540.
In the actual transmission circuit, signal transmission is performed in a stable state such that the maximum potential on the Pos line is substantially equal to that on the Neg line, and the minimum potential on the Pos line is substantially equal to that on the Neg line. As a result, the differential potential changes up and down about 0 V.
The analysis results shown in FIG. 11, however, are from a halfway state before the circuit model 500 enters the steady state. Line P1 indicating the change in the potential on the Pos line model 530 is on the high-potential side (the upper section in the Waveform) with respect to line N1 indicating the change in the potential on the Neg line model 540, as shown in part (A). Line D1 indicating the change in differential potential changes up and down about a potential of 300 mV much higher than 0 V, as shown in part (B).
With the lapse of the analysis time, the potential on the Pos line model 530, the potential on the Neg line model 540 and the differential potential in the simulation become closer to the values in the actual signal transmission. However, a long analysis time is required to obtain the desirable results, as described.
For transient analysis in a transmission circuit using an AC coupling component such as that described above, a method is conventionally adopted in which a suitable potential for enabling a circuit model representing the transmission circuit to reach a steady state in a reduced time period is given to the circuit model as an initial value in transient analysis in advance. However, a certain level of skill is required to obtain and give a suitable initial value.